Retrait-gonflement d'un matériau argileux exposé aux cycles d'humidification/séchage

Cet article présente les résultats d'études expérimentales effectuées au laboratoire sous différents chemins de contraintes hydrique et mécanique sur un matériau gonflant compacté. Plusieurs cycles de variation de potentiel hydrique compris entre 0 et 8 MPa ont été appliqués sur des éprouvettes sous différents chargements mécaniques constants (15, 30 et 60 kPa). Les éprouvettes manifestent un retrait cumulé pendant ces cycles, et les déformations convergeant vers un point d'équilibre. L'ensemble de ces résultats expérimentaux a été interprété par le modèle élastoplastique du sol gonflant BExM proposé par Alonso et al. (1999) qui prend en compte l'accumulation des déformations lors de l'application de différents chemins de chargement. La correspondance entre les résultats mesurés et calculés est satisfaisante pour chacun des chemins suivis

Influence of suction cycles on the soil fabric of compacted swelling soil

International audienceThe soilfabric plays an important role in complex hydromechanical behaviour of the expansive soils. This article addresses the influence of the wetting and drying paths on the soilfabric of compacted bentonite and silt mixtures at two different initial dry densities corresponding to loose and dense states. To obtain the hydric response of the soil, two suction imposition techniques were used: osmotic technique for the suction range less than 8.5 MPa and the vapour equilibrium or the salt solution technique for the suction range between 8.5 and 287.9 MPa. Additionally, the soilfabric analysis was performed using mercury intrusion porosimetry (MIP) and nitrogen gas adsorption (BET) techniques. The dense samples produced cumulative swelling strains during the suctioncycles, while shrinkage was observed for the loose samples. The suctioncycles induced an equilibrium state indicative of the elastic behaviour of the samples. The soilfabric analysis showed that regardless of the soil's initial state (loose or dense), the samples obtained the same soilfabric at the equilibrium state. The experimental results illustrated also the existence of an elastic void ratio (e0el) where the compactedsoils at this state present an elastic hydric behaviour during the successive suctioncycles

Unsaturated resilient behavior of a natural compacted sand

Granular materials are generally used in unbound layers of road pavement structures. The mechanical behavior of these materials is widely studied with repeated load triaxial tests (RLTT) in which the elastic response is defined as the resilient behavior. Usually observed under total stress conditions, the effect of pore pressure changes during loading are not usually included in design. Further, the unbound layers frequently exist under partially saturated conditions. The influence of the unsaturated state, i.e., the suction, on the mechanical behavior, of unbound granular materials for roads has not been sufficiently studied and is generally not taken into account in models used for these materials. This article presents an experimental study of the repeated load response of a compacted clayey natural sand, and describes a model for the response which includes the effects of soil suction. The response of the proposed model formulated in terms of effective stress is compared with a similar model formulated in terms of total stress. The results from both the effective stress model and the total stress model are compared with the measured volumetric and deviatoric response. It is suggested that since the model parameters for the effective stress formulation are relatively constant for all values of suction (water content), the resilient response can be best captured by an effective stress model

Seasonal thermal energy storage in shallow geothermal systems: thermal equilibrium stage

This paper is dedicated to the study of seasonal heat storage in shallow geothermal installations in unsaturated soils for which hydrothermal properties such as degree of saturation and thermal conductivity vary with time throughout the profile. In the model, a semi-analytical model which estimates time-spatial thermal conductivity is coupled with a 2D cylindrical heat transfer modeling using finite difference method. The variation of temperature was obtained after 3 heating and cooling cycles for the different types of loads with maximum thermal load of qmax = 15 W.m−1 with variable angular frequency (8 months of heating and 4 months of cooling).and constant angular frequency (6 months of heating and 6 months of cooling) to estimate the necessary number of cycles to reach the thermal equilibrium stage. The results show that we approach a thermal equilibrium stage where the same variation of temperature can be observed in soils after several heating and cooling cycles. Based on these simulations, the necessary number of cycles can be related to the total applied energy on the system and the minimum number of cycles is for a system with the total applied energy of 1.9qmax

Comportement hydromécanique des matériaux granulaires compactés non-saturés

Ce travail présente l'effet de la non-saturation sur le comportement réversible mécanique des matériaux granulaires de chaussées à court terme. On commencera par caractériser le comportement hydrique des différents matériaux. A partir des matériaux précédents, on caractérisera à la boite de cisaillement direct (à chargements statiques) et à l'appareil triaxial (à chargements répétés) les comportements réversibles. Ce travail nous permettra finalement d'injecter la succion dans les modèles classiques d'élasticité non linéaire pour le comportement mécanique des sols granulaires saturés

Shakedown modeling of unsaturated expansive soils subjected to wetting and drying cycles

It is important to model the behavior of unsaturated expansive soils subjected to wetting and drying cycles because they alter significantly their hydro-mechanical behavior and therefore cause a huge differential settlement on shallow foundations of the structure. A simplified model based on the shakedown theory (Zarka method) has been developed in this study for unsaturated expansive soils subjected to wetting and drying cycles. This method determines directly the stabilized limit state and consequently saves the calculation time. The parameters of the proposed shakedown-based model are calibrated by the suction-controlled oedometer tests obtained for an expansive soil compacted at loose and dense initial states, and then validated for the same soil compacted at intermediate initial state by comparing the model predictions with the experimental results. Finally, the finite element equations for the proposed shakedown model are developed and these equations are implemented in the finite element code CAST3M to carry out the full-scale calculations. A 2D geometry made up of the expansive soil compacted at the intermediate state is subjected to successive extremely dry and wet seasons for the different applied vertical loads. The results show the swelling plastic deformations for the lower vertical stresses and the shrinkage deformations for the higher vertical stresses

Resin injection in clays with high plasticity

International audienceRegarding the injection process of polyurethane resins in clays with high plasticity, this paper presents the experimental results of the pressuremeter and cone penetration tests before and after injection. A very important increase in pressure limit or in soil resistance can be observed for all the studied depths close to the injection points. An analytical analysis for cylindrical pore cavity expansion in cohesive frictional soils obeying the Mohr–Coulomb criterion was then used to reproduce the pressuremeter tests before and after injection. The model parameters were calibrated by maintaining constant the elasticity parameters as well as the friction angel before and after injection. A significant increase in cohesion was observed because of soil densification after resin expansion. The estimated undrained cohesions, derived from the parameters of the Mohr–Coulomb criterion, were also compared with the cone penetration tests. Globally, the model predictions show the efficiency of resin injection in clay soils with high plasticity

Equilibrium Stage of Soil Cracking and Subsidence after Several Wetting and Drying Cycles

This work investigates the equilibrium stage of the crack propagation of a fine-grained soil after several drying and wetting cycles (shrinkage and swelling hysteresis). This stage is found to be crucial in practical engineering since the soil continues to show its irreversible hydraulic settlement, which is a potential risk for some severe structural damages. The shrinkage area and the shrinkage crack area were determined by using the image processing method. For the cyclic experimental investigations, the shrinkage cracks were followed during six months of successive wetting and drying cycles for two samples (with two different initial water contents). These long-term tests were completed by some short term single drying path tests performed on samples prepared at different initial states. The results showed the existence of a unique equilibrium stage at the end of the wetting and drying cycles for the two studied samples. The equilibrated soil subsidence was separated into two parts: the reversible settlement of the equilibrium stage and the irreversible settlements cumulated during successive wetting and drying cycles. At the equilibrium stage, the reversible deformation was 5.9% and the irreversible deformation was 3.8%. A simplified theoretical approach was also used to predict the cracking equilibrium stage and its soil subsidence. The fitted parameters of the theoretical approach for each cycle were stabilized to confirm the existence of this equilibrium stage